The present embodiments relate to positron emission tomography (PET).
PET systems and corresponding detectors have a limited field of view. Typically, the entire patient cannot be scanned with the patient in one position. In a step and shoot (S&S) approach to scan a greater portion of the patient, the patient is moved between sequential scans, but remains stationary for each of the scans. However, the gaps between blocks of detectors cause a drop in axial sensitivity when assembling the different scans for segment zero (2D) acquisitions. For three-dimensional (3D) acquisition, greater axial uniformity may be provided for S&S. Low sensitivity spots of various segments are mixed in image space, and therefore the loss of sensitivity only occurs in the end planes.
Continuous bed motion (CBM) acquisition performs a scan of the patient while the patient is moving through the PET system. CBM may improve the axial uniformity of PET images over S&S for 2D acquisitions. For 3D acquisitions, CBM acquisition may result in super-resolution images by oversampling the image in the axial direction.
Normalization may be incomplete for CBM. The PET model represents data as a line integral (Rf) of object activity, corrected for detection efficiency:
                                          p            _                                i            ,            t                          =                                            1                              n                i                                      ⁢                        ⁢                          f              i                                =                                    1                              n                i                                      ⁢                                          ∑                j                            ⁢                                                          ⁢                                                C                                      ij                    ,                    t                                                  ⁢                                  f                  j                                                                                        (        1        )            where C is the system matrix with the spatial projection index i and TOF bin index t, f is the object emission activity image, identified at image voxel with index j, and n is a normalization coefficient, which represents the inverse of efficiency for a given detector pair. The detector pair defines the line of response (LOR). In the current clinical S&S mode, the normalization coefficient is decomposed into the geometrical component, crystal efficiencies, and sinogram plane efficiencies. Crystal efficiencies are corrected for dead time when the singles rate per detector is known. Decay correction, an activity efficiency factor, is applied in the image domain in S&S reconstruction, outside of the normalization coefficients. In S&S, the normalization coefficients may be computed as in any PET scan. For CBM, the continuous motion of the patient may result in inaccuracies in the PET model for the normalization coefficient.